1. Field of the Invention
The present invention refers to a glass windshield which has a layer of transparent coating on its surface, providing integrated capabilities as a protective screen against the sun, reducing radiation inside the vehicle, and as an image combiner.
At present, it is possible to see the running state of the vehicle and the driving conditions through the panel of instruments located behind the steering wheel, under the windshield.
However, in order to read the symbols showing the various data, the driver is required to lower his eyes, thus compromising driving safety. A system known in the art as "head-up display" allows the projection and thus the visualization of certain items of control data for the vehicle in the driver's field of vision, superimposed on the external view: This system increases the degree of driving safety.
2. Description of the Prior Art
A head-up display system is formed by four basic parts: a source of bright light, apt to display in various forms, such as numeric, alphanumeric or graphic, the information to be shown to the driver;
an electronic part, formed by a microprocessor system to process data and provide the information to be visualized;
an optical system which carries the beam of light and enlarges and focuses the image at a certain distance from the eyes of the driver;
a combiner which superimposes the image of the driving information on that of the outer environment.
Said parts are assembled and installed on a motor vehicle and constitute an integral part thereof, as does the instrument panel.
Head-up displays are known from the prior art.
In U.S. Pat. No. 4,806,904 to Nissan Motor Co. there is described a head-up display which displays the speed of the vehicle on the windshield. The combiner is a sol-gel mixture of SiO2-TiO2 applied directly on the inside of the windshield, on a restricted area impinged by the light beam emitted from the source. The source of bright light is a numerical display of the VFD (vacuum fluorescent display) type.
U.S. Pat. No. 4,740,780 to Gec Avionics Inc. describes a head-up display capable of displaying numeric, alphanumeric and graphic messages and the combiner is a conventional untreated windshield. The source of bright light is an array of pixels formed by squared gallium phosphide LEDs, which provides a good brightness.
The publication "A holographic head-up display for automotive applications" of Flyght Dynamics Inc. on Proceedings of S.P.I.E. vol. 958 of 1988 describes a head-up display capable of visualizing images on the windshield of a motor vehicle, where the combiner is formed by a volume hologram, transcribed onto dichromate-additioned gelatin on a polyester support, and inserted, after being transcribed, on the windshield under autoclave. The hologram has no optical capacity and functions as a plane mirror. The source of bright light is formed by a halogen tungsten lamp and by a panel of transmitting liquid crystals covered by a solar filter.
European patent application n. 229876 of Yazaki Corporation describes a head-up display which displays standard information necessary to the driver, utilizing the untreated windshield as the combiner. The source of bright light is formed by an incandescence lamp placed behind a panel of transmitting liquid crystals.
The publication "A head-up display for automotive use" on Proceedings of SIB, vol. 28/3 of 1987 of Mitsubishi Electric Corp. describes a head-up display which displays the images on a windshield which has been treated with a thin layer of aluminium, evaporated on the inside surface of the same windshield. The source of bright light is formed by a fluorescent lamp emitting polarized light.
French patent application number 2569863 describes that a small area of a windshield for use in a train or a car can be coated with a TiO.sub.2 mono-layer, to obtain a head-up display. The light transmission value in the above mentioned area is stated to be about 55%. As a value of 55% for light transmission through a windshield is far below that required for a satisfactory transmission quality (as a comparison, the EEC standards require a value no lower than 75%), the teaching of this prior art document would appear to be contrary to using a TiO.sub.2 mono-layer on the whole surface of a windshield for a head-up display purpose.
German patent application No. 3828137 of Central Glass Co. Ltd. describes a process to form a coating film of TiO.sub.2 on a substrate with selected optical reflection characteristics. The TiO.sub.2 layer is applied by a chemical process. This involves the obtaining of layers of a thickness ranging from 2900 to 3100 .ANG.. Such thickness does not allow the use of a TiO.sub.2 layer on the whole surface of a windshield, as it involves a light transmission value of too low a level.
European Patent application 219273 describes a multi-layer transparent article having a high visibility transmittance. This document is directed to obtaining a sheet of glass having satisfactory luminous transmission characteristics with a coating suitable to an electrically heated windshield. The reflecting characteristics of this glass, however, are apparently depressed by anti-reflective ZnO layers, so that a use of the glass in a head-up display is to be excluded due to insufficient reflective capabilities.
It is furthermore known from the art that, to define the quality of a head-up display, it is necessary to consider principally the optical characteristics of the device and particularly the quality of the image generated by the device.
In more detail, as the quality ratio of an image the parameter Qr=L.sub.i /L.sub.f is defined, where L.sub.i is the brightness of the image and L.sub.f is the brightness of the background, that is of the outer environment. Clearly, the higher this ratio, the higher is the quality of the image, and therefore the higher are the optical characteristics of the head-up display. It is also known that to obtain a good image brightness, that is a high value of L.sub.i, one needs not only to have a high value of the source brightness, but also a high reflecting value of the combiner.
It is in fact possible to calculate the value of Qr by the following formula: ##EQU1## where D is the luminance of the light source, m.sup.2 is the background luminance, T.sub.o is the optical transmission spectrum (except the combiner windshield), m is the total linear enlargement of the optical system, R is the reflection value of the glass, referred to the light incident on the inside of the vehicle with a certain angle of incidence, T is the value of the light transmission referred to the light coming from the outside of the vehicle, with a certain angle of incidence.
A construction of a head-up display for motor vehicles is subjected to the following restrictions. The light transmission of the windshield must not be lower than 75%, according to EEC standards, or 72% under USA standards. The combiner must provide an index of color fidelity of over 90%. A low cost source of light must be used which is compatible with the overall cost of the motor vehicle.
Moreover, it is required that the glass for motor vehicles also serve the purpose of sun screen, both when it is used as a windshield, and when it is used as a rear or side window.
A glass is considered to function as a sun screen when it allows no more than 55% of the perpendicularly incident energy to be transmitted into the cabin.
In view of the contrasting optical requirements and the limitations described above, it is verifiable that, under direct solar radiation, the image results poorly visible due to the relatively low luminance of the source compared to that of the background, as a consequence of the restriction to a light source of a brightness that is neither very strong nor, in particular, very costly. In fact, although sources of light such as cathode tubes (CRT), which are capable of providing a high luminance, are commercially available, their high cost and excessive size are not compatible with the overall cost and dimensions of a motor vehicle.
Moreover, if a sheet of glass, particularly a windshield, is treated to increase its reflectance and therefore improve the luminance of the image, its transmittance of light becomes reduced below 75%, a reduction which is not commercially acceptable.
With the use of a hologram, as explained also in the prior art, there is a risk of low durability, together with the fact that it becomes difficult and costly to insert and assemble said hologram between two double curvature glass sheets.
In any case, from the prior art there does not appear to be any windshield capable of functioning with its entire surface and simultaneously as a sun screen and a combiner for a head-up display. This is nevertheless what would be sought after by motor vehicle manufacturers.